Mesh fixes

Two things:

  - HOTFIX: `trilist` is optional. It wasn't optional for `TexturedTriMesh`, but in recent changes I made it consistent with `TriMesh`. However, the importer tests didn't flag that the meshes were being built in the
wrong order.
  - CHANGE: `tcoords` is now a plain `ndarray`. I think this makes more sense as the rest of the objects on a `TexturedTriMesh` are `ndarrays`. I can't see the use case for ever needing to use the `PointCloud` abilities on `tcoords`?

Obviously, I added tests to ensure these things are the case!